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sediment cores may be functions of both fire frequency and severity, with concen-
trated charcoal layers (or charcoal peaks) in the time series reflecting either
individual high severity events, frequent fire periods, concentrated erosion
periods, or all of these processes in combination. These studies have shown
vegetation changes in concert with changes in climate and fire (Whitlock et al.
2004 ; Power et al. 2008 ; Marlon et al. 2009 ). Although these studies have sampled
over a broad regional and global scale, they are biased toward reflecting fire
regimes on landscapes with Holocene lake deposits. On some landscapes these
are primarily high-elevation or high-latitude areas that are characteristic of conifer
forests with high-frequency surface fire regimes. One of the conclusions drawn
from these studies is that Holocene climate was the only important driver of fire
regimes and humans had little impact. In lightning-saturated environments this
was probably true. However, many of the landscapes not represented in the
charcoal record are non-forested environments where lightning ignitions are
limiting and Holocene population densities were high, as were human sources of
ignition (Keeley 2002b ). Recent charcoal studies in the Mediterranean Basin have
detected clear signs of both climate and humans affecting Holocene fire frequency
(Gil-Romera et al. 2010 ).
Fire Patch Size and Distribution
Fire size varies over many orders of magnitude, from a lightning-ignited fire
that remains localized around the tree it strikes, to massive crown fires that
burn hundreds of thousands of hectares. On some landscapes a small percent-
age (5% or less) of fires account for 95% of the area burned (Strauss et al.
1989 ). This means that it is primarily the very large fires in the tail of the size
distribution that determine the age distribution and spatial age mosaic of the
landscape.
Distributions of fire size vary regionally and between surface fire and crown fire
regimes ( Fig. 2.2 ). Within fire perimeters the size of different fire severity patches
may vary greatly, creating a mosaic of patches ( Fig. 2.3 ). Many forests exhibit
complicated patterns of fuel consumption, comprising a mixture of surface fire,
crown fire and unburned patches. This heterogeneity is important to ecosystem
processes such as tree recruitment, which often requires gaps in the forest canopy
coupled with patches of surviving parent seed trees (see Chapter 3 ). Fire-induced
gaps in the forest canopy provide high light environments for successful seedling
establishment, but equally important they also accumulate surface fuels at a
slower rate, and thus contribute to patchiness of burning in subsequent fires,
providing safe sites for saplings until they reach sufficient size to survive fires
(Keeley & Stephenson 2000 ).
MTC shrublands commonly experience large crown fires that cover vast areas
and often in a rather coarse-grained pattern of uniform high severity. This poses
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